In recent years, as a fastening device for disposable paper diaper, medical simple clothing, napkins, various kinds of working simple clothing, underwear and the like, the surface fasteners each having a number of engaging elements of minute dimension molded integrally with a flat base material by continuous injection molding of thermoplastic resin have been often used. The configuration of the engaging element of the integrally molded surface fastener of this kind is classified largely to hook type and mushroom type as well known in the related field.
Typical hook type integrally molded surface fasteners have been disclosed in, for example, U.S. Pat. No. 4,984,339 and U.S. Pat. No. 5,537,723. In case of these molded surface fasteners, molten resin is supplied to the circumferential face of a cylindrical drum in which a number of substantially letter J shaped cavities are formed such that they are open to the circumferential face in a curved configuration from inside of the drum so as to mold a thin base material along the circumferential face while at a time, an inverted letter J shaped hook is molded integrally with the rear face of the base material by filling the cavity with part of molten resin. After cooling, a molded product is produced by peeling it from the circumferential face of the drum.
An integrally molded surface fastener having a modified configuration of the aforementioned hook type engaging element has been also disclosed in, for example, Japanese Patent Laid-Open Publication No. 2-5947 and Japanese Patent Laid-Open Publication No. 6-133808. Upon production of the molded surface fastener, molten resin is extruded from an extrusion die having a number of openings erected in the form of substantially two leaf palm tree or letter T on a narrow slit extending horizontally, so that a number of rows of ribs having palm tree shaped or letter T shaped section in the extrusion direction are molded continuously on a thin base member so as to produce a primary molded product. Next, the rib of the primary molded product is cut out successively by a predetermined thickness along the length direction and then, palm tree shaped or letter T shaped engaging elements are fit thereto. After that, the base member is extended in the molding direction so as to separate the respective engaging elements by a predetermined pitch so as to product a molded surface fastener having a final configuration.
More recently, molded surface fasteners having a novel hook configuration in which the aforementioned hook type engaging element configuration is modified largely have been developed. Those molded surface fasteners have been disclosed in, for example, Japanese Patent Laid-Open Publication No. 9-322812 and Japanese Patent Laid-Open Publication No. 11-56413. The engaging element of these publications comprises an erecting portion which erects upward from the surface of the base member, a neck portion which separates in the molding direction with a V-shaped gap at the top end of the erecting portion and engaging head extending substantially horizontally back and forth in the molding direction from each neck portion. Further, the bottom face of the engaging head is inclined obliquely upward as it goes toward the front end and its top face is a flat plane and in a front view along the molding direction, expanded portions each having a predetermined thickness in the vertical direction are provided on the right and left sides of the top face.
As its basic production method, the primary molding technology using the aforementioned cylindrical drum has been adopted. However, the engaging element molded on the surface of a base member upon molding is not molded in the form of letter J from the beginning, but in a side view along the molding direction, a primary engaging element material having two V-shaped or four cross-shaped branches extending upward in an inclined state at the top end of its erected portion is molded preliminarily, so that a preliminarily molded surface fastener in which a number of the primary engaging element materials are molded integrally on the base member is produced. Then, by pressing the branch portions of the primary engaging element materials of the preliminarily molded surface fastener with heat, the top portion is softened and deformed to mold engaging elements having peculiar configuration, which the conventional art has never seen.
On the other hand, the mushroom type molded surface fastener has been also well known since before and according to for example, U.S. Pat. No. 3,192,589, after the preliminarily molded surface fastener in which a number of pillars are erected on the surface of the base member is molded, the front ends of the pillars of the molded product is softened by heating so as to mold semi-spherical engaging heads for manufacturing of the molded surface fastener. For example, Japanese Patent Laid-Open Publication No. 8-508910 has disclosed an improvement technology of the mushroom type molded surface fastener. According to the molded surface fastener disclosed in this publication, the front ends of the pillars of the primary molded product are softened by pressing and heating so as to mold circular plate engaging heads instead of the semi-spherical engaging heads.
Because the aforementioned mushroom type engaging element engages in all directions, it has been known that it has stronger engaging strength and separation strength than the hook type engaging element whose engagement has a directivity. However, because in the engagement mechanism of the pile to the mushroom type engaging element, the pile engages in so-called hanging condition that it winds up around the neck portion, if it is intended to release the pile from the engaging head, the probability that the engaging element may be cut out or the pile itself may be cut out at the neck portion is high, so that this often cannot bear repeated usage and at a time, the engaging strength and separation strength likely increase excessively.
The molded surface fastener disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 8-508910 is an improvement based on the advantages and disadvantages of the mushroom type and intensifies the engagement ratio to minute piles by forming the shape of the engaging head into a substantially circular shape while securing an appropriate separation strength. This can be adopted as a fixing device to various kinds of unwoven minute piles, which are attached to the aforementioned disposable diaper. The application of such fixing device has been expanding rapidly as described above, and a further improvement has been still proposed in, for example, International Publication No. 98/57565 pamphlet. That improvement aims at intensifying the plasticity of the engaging head by forming much minute unevenness on the top face of the circular engaging head.
On the other hand, as compared to the mushroom type engaging element, the aforementioned hook type engaging element is more likely to engage the pile and further, obtain appropriate engagement strength and separation strength. At the time of separation, the engaging elements and piles are never cut out and can bear repeated usage. In addition to the aforementioned Japanese Patent Laid-Open Publication No. 6-133808 and Japanese Patent Laid-Open Publication No. 9-32281, for example, Japanese Patent Application No. 2001-64460 filed by these inventors has proposed an integrally molded surface fastener to adopt such advantages of the hook type for the fixing device of the disposable diaper and the like.
This integrally molded surface fastener is a molded surface fastener having minute engaging elements of thermoplastic resin which are molded together with the base member by continuous molding. The engaging element is composed of a single pillar portion comprising first and second pillar portions which intersect with each other at right angle having a substantially cross shaped section. The pillar portions extend in opposite directions to each other in a wing-like form along the intersection directions of the first and second pillar portions with respect to the top end of the pillar portion and have a thin engaging head having substantially the width as the width of the top end of the second pillar portion.
If such minute engaging elements having the peculiar head configuration are molded, securely they engage fiber piles planted densely and individual engaging elements can secure appropriate engaging force, shearing force and separation force and at a time, the tactile feeling of the surface fastener is improved. The height of the engaging element projecting from the surface of the base member can be reduced as compared to conventional type. On the other hand, because the sectional shape of the pillar portion is formed into a cross shape as described above, the engaging element is blocked from falling down against a pressing force so as to secure a high engagement ratio with mating pile pieces. Consequently, a durability demanded to this kind of the fixing device is satisfied and desired plasticity and shearing strength of the flat base member can be secured.
According to the production technology of the hook type molded surface fastener disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 6-133808, a number of ribs erected on the base member of a preliminarily molded surface fastener molded primarily are cut out successively by a predetermined pitch along the length direction and its base member is stretched in the length direction (molding direction) so as to separate individual engaging elements to a predetermined interval. Therefore, the thickness in the molding direction of the engaging element is determined by a cutting pitch and its stiffness is determined by the material and cutting thickness of used resin. Thus, if it is intended to obtain engaging elements which engage/disengage minute piles like for example, unwoven cloth surface according to such a production method, naturally the thickness in the molding direction becomes very small, so that they are likely to buckle in the molding direction. Thus, the stiffness cannot be improved until the thickness is increased.
In case of the molded surface fastener having the hook type engaging elements based on the aforementioned Japanese Patent Laid-Open Publication No. 9-322812 or Japanese Patent Laid-Open Publication No. 11-56413, although the fall-down of the erected portion decreases and the engagement ratio is secured because of the peculiar shape of the engaging element, necessary separation strength is more difficult to obtain as the minuteness progresses and increase in the separation strength to the disposable diaper and the like has been strongly demanded as described above. As compared to the engaging element according to the Japanese Patent Application No. 2001-64460, although a high engagement ratio with a mating pile piece is secured by preventing the fall-down of the engaging element against a pressing force, so that the durability required for this kind of the fixing device is satisfied and desired plasticity and shearing strength of the flat base member can be secured, it cannot be said that its engaging force and separation strength are sufficient.
On the other hand, in case of the molded surface fastener having the mushroom type engaging element disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 8-508910 or International Publication No. 98/57565 pamphlet, despite its minuteness, the root portion having an arbitrary size can be molded to provide with a required stiffness which prevents its buckling. However, if it is intended to provide the root portion with a desired stiffness, the size of the root portion cannot help being increased.
If the diameter of the root portion increases, even if the extending length of the engaging head which extends from the root portion to all directions is to be minute, necessarily the diameter of the engaging head increases by the amount corresponding to the increase in the diameter of the root portion, so that no minute engaging element can be molded and if it is molded in a large size, naturally it is difficult to engage a loop. Further, even if the engaging element is molded in a minute size, the so-called hanging, which occurs in the mushroom from the peculiar configuration, cannot be avoided and it is cut out easily between the pillar portion and the engaging head or the mating pile is cut out easily. Thus, it is difficult to secure a necessary durability.
As for a problem which this kind of the integrally molded surface fastener faces, if it is used as a disposable paper diaper, medical simple clothing, napkins, various of working clothing, underwear and the like, an engaging object becomes extremely minute piles exposed on the surface of ordinary unwoven cloth or knitted fabric, necessarily, the size of the engaging element of the integrally molded surface fastener needs to be small and particularly, because there is much opportunity that it makes contact with soft skin of infant, the engaging element needs to have mild tactile feeling as well as plasticity. Further, this engaging element needs to bear repeated usage of two or three times sufficiently although it is disposable and at a time, the separation strength needs to be high enough for the diaper not to loose out easily.
In case of a conventional integrally molded surface fastener, the engaging element can be molded in minute sizes, and for example, in case of the mushroom type engaging element, its separation strength is too high so that the minute pile or engaging element itself is likely to be damaged at the time of separation. In case of the hook type engaging element, there is a problem that a desired separation strength cannot be secured although it never damage the minute piles or engaging elements itself.